This past weekend I was in Austin, speaking at the MomCom conference. While I was there, a lovely woman who was familiar with my work approached me, and we started talking. At one point, she mentioned that her daughter had recently bought an SLR camera, and asked what single piece of advice I would give her.
Because I'm difficult, I instead came up with two things:
1. Shoot every day. Like, seriously, every day, without fail. Even if it's the end of the day, and you're tired and realize that you never took your shot, just grab your camera and compose a shot wherever you are -- 3 feet from your bed, if necessary. But shooting every day serves two purposes: first, it forces you to become creative with your compositions and learn new ways of using your camera; second, it gives you a body of work to monitor progress over time.
2. Periodically put your photographs somewhere where people can see them. Online, preferably. Not just for the feedback -- I mean, of course feedback is fine -- but more for the fear of feedback. When you shoot knowing that other people will see your work, you try harder. It's a good motivator to keep improving your work.
All the other stuff -- learning about ISO, shutter speed, aperture, composition, post-camera processing -- it'll come: you'll be motivated to figure it all out, either by taking classes, or going online, or simply experimenting on your own. But do those two things above, and you will inevitably get better.
(While I'm pretty good at #2, I'm not as good as I used to be with #1 -- something I realized while I was walking around my neighbourhood yesterday, taking the shots above. Time to return to keeping my camera on me every day, I think ...)